Lack of fusion (LOF) defects impact adversely on the mechanical properties of additively manufactured components produced via laser-based powder bed fusion. Following a stress-relieving heat treatment, the tensile properties and hardness of Ti6Al4V components were found to be negatively impacted by the presence of LOF defects. This work considers a geometrical-based inequality for the prediction of LOF defects. We critically evaluate an LOF criterion using both the experimentally and analytically obtained melt pool geometries. Experimentally, we determined melt pool dimensions by analysing a single-layer, multi-track deposition with oversized hatch spacing in order to establish depth and width from non-overlapping melt pools. Analytically, Rosenthal-based predictions of melt pool size (width and depth) are applied. To investigate LOF defects, we used hatch spacing as the main parameter variation to investigate defects while keeping all other controllable parameters unchanged. An original LOF criterion from the literature was found to be an adequate predictor of LOF defects when experimentally obtained melt pool geometry was used. Critically, however, the analytical expressions for melt pool geometry were found to be in error and this caused the LOF criterion to fail in predicting LOF defects in all cases where defects were observed experimentally. However, an adaptation to the LOF prediction criterion is proposed whereby it is recommended that a correction factor R-c(2)=0.7 (or R-c=0.83) is used with the analytically derived melt pool geometry. Furthermore, this correction is extended into the laser power versus scanning speed operating space to give minimum (corrected) line energy for LOF avoidance in Ti6Al4V components.

The properties of Extra Low Interstitials (ELI) Ti6Al4V components fabricated via the laser-based powder bed fusion (L-PBF) process are prone to variation, particularly throughout a powder reuse regime. Interstitial pick-up of interstitial elements within the build chamber during processing can occur, most notably, oxygen, nitrogen, and hydrogen, which can impair the mechanical properties of the built component. This study analyses ELI Ti6Al4V components manufactured by the L-PBF process when subjected to a nine-stage powder reuse sequence. Mechanical properties are reported via hardness measurement and tensile testing. Results showed that from 0.099 wt.% to 0.126 wt.% oxygen content, the mean hardness and tensile strength increased from 367.8 HV to 381.9 HV and from 947.6 Mpa to 1030.7 Mpa, respectively, whereas the ductility (area reduction) reduced from around 10% to 3%. Statistical analysis based on the empirical model from Tabor was performed to determine the strength–hardness relationship. Results revealed a significant direct relationship between tensile strength and Vickers hardness with a proportionality constant of 2.61 (R-square of 0.996 and p-value of 6.57 × 10−6).

Additive manufacturing (AM) is defined as a technology performed for tooling applications. It is used for manufacturing tools that have complex shapes and figures. In this study, an extensively applied Ti-6Al-4V alloy was made using the selective laser melting method. Post-production heat treatments were applied to decrease thermal stresses and to enhance the mechanical properties and the microstructure. The study investigates the fatigue mechanical properties, microstructure, hardness, and porosity of the AM Ti-6Al-4V after stress relieving (SR) and after SR followed by hot isostatic pressing (HIP). The samples’ upper and lower parts were independently examined to determine the effects of thermal conditions and the heat treatment of the microstructure. The microstructures were examined through optical microscopy, scanning electron microscopy and x-ray diffraction methods. The mechanical properties were investigated through microhardness testing, alongside assessment by fatigue testing at room temperature. The findings demonstrated that the microstructure after SR at 704 °C for 2 h is 100% fine martensitic α '-Ti, with a microhardness value of 408 HV. Air and furnace cooled samples have a more homogenous structure and are characterised by mixture ( α + β ) with microhardness values of 382 and 356 HV, respectively. After HIP at 920 °C and 100 MPa for 2 h was applied, the martensite was converted into a lamellar ( α + β ) microstructure, whereby the α phase is presented as fine needles situated among the β ridges in the microstructure, with the existence of the prior β grain boundary.